Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 3:21 PM GMT on November 08, 2006
In my blog last week, I discussed Thingamabobbercane, an oddball cyclone which had characteristics of both a tropical cyclone and extratropical cyclone. I referred to it as a subtropical storm, which the Glossary of Meteorology defines thusly: A cyclone in tropical or subtropical latitudes (from the equator to about 50°N) that has characteristics of both tropical cyclones and midlatitude (or extratropical) cyclones. I gave the storm the name Thingamabobbercane because although it fit the technical definition of a subtropical storm, it didn't look like a typical subtropical storm. In particular, it formed at 41 N latitude over very cold waters of 16-18C, which is unusually cold and far north for a subtropical storm. Well, there are some meteorologists who disagree with my classification of the storm as subtropical, and believe Thingamabobbercane had virtually no tropical characteristics. A detailed day-by-day analysis posted on storm2k.org argues that Thingamabobbercane was an extratropical cyclone that underwent a process known as warm seclusion. In the seclusion process, a strong extratropical cyclone draws in warm air from the south, and latent heat of condensation from the cyclone's intense precipitation makes this air even warmer. This extra-warm air spirals into the center of the low and wraps around to the west side, where it is pinched off. As result, one has an isolated "warm core" center where deep convection builds and spiral banding can occur. However, unlike a hurricane, there is no eyewall, and no cloud-free eye created by sinking air (subsidence) in the center. The eye-like feature in an extratropical cyclone undergoing warm seclusion has upward moving air, and is merely the center where the surface winds spiral into. Spiral bands of convection can develop in the warm air near the center, mimicking the spiral bands of a hurricane. If these convective bands become intense, subsiding air on the flanks of the bands may create subsidence that warms and dries out the surrounding air, creating cloud-free regions near the center that may give it a more eye-like appearance. I flew through a number of these type of systems in 1989 as a member of the ERICA field program, and noted in my blog on the Blizzard of 2006 that the storm had a warm seclusion.
Figure 1. Image of Thingamabobbercane from Nov. 1, 2006, taken by NASA's Terra satellite. Image credit: NASA's Earth Observatory web page.
Steve Gregory, who did an awesome job blogging for wunderground.com during last year's unbelievable hurricane season, and has moved on to form his own hurricane consulting firm for business and industry, weatherinsite.net, had this to say about Thingamabobbercane:
I think that just having a transitory very low level warm core doesn't warrant a system being called sub-tropical. We clearly do not want to name a Nor'easter simply because it may form a low level warm core and 'eye' for a day or two. These super storms, or 'Perfect Storms', usually get their very own 'names' anyway ("The Great Blizzard of 78', etc). I think most everyone agrees that sub-tropical storms evolve out of old occluded extra-tropical lows that typically drift southward, completely isolate themselves from any frontal systems, and start becoming warm core--and usually over water above 20°C or warmer. And, they have life spans of many days as they transition, and they either remain sub-tropical or go all tropical with absolutely no cold or warm fronts even remotely close to them.
Whereas these briefly lived specimens, especially when at such high latitudes (over 40N) and truly cold water (15 deg is cold), never really fully detach from frontal boundaries, do the classic counterclockwise track motion under the upper level 500mb low embedded, all of which is embedded within full latitude long wave TROFS--and then transition quickly into non-tropical systems that generate new frontal based surface lows--just shouldn't be called sub-tropical. And especially if the warm core remains below 5,000 ft.
We need a new classification for these types of half breeds that get some of their energy through the latent heat release process etc, but are 'clearly' not what we think of as sub-tropical storms that will either stay that way, or go all tropical, or after a few days, get picked up by an upper TROF and pulled north where they transition back to a fully non-tropical low.
And here is what Senior Hurricane Specialist James Franklin from the National Hurricane Center had to say about the system in an email I received:
The system was of frontal origin, that much is clear. But I believe the frontal structure was eventually lost (no way to know for sure). The convective structure resembled a tropical, rather than subtropical cyclone, and the radius of maximum winds (based on QuikSCAT) was very close to the center, also more typical of tropical cyclones. It was, for most of its existence, under an upper low, typical of subtropical cyclones. However, it was developing a modest mid to upper lever warm core, moving toward tropical structure. So structurally, on balance, it was more tropical than subtropical.
However - it was over sub 18C water, and part of the definition of a tropical cyclone is that it originates over tropical or subtropical waters. This one didn't, so it's not a tropical cyclone by our operational definition, even though it had some of the characteristics of one.
Our classification system is a convenience for man, but Nature is not the slightest bit interested in our classifications of cyclones. There is a complete spectrum of storms between extratropical and tropical. There are cyclones that have similarities to tropical cyclones in structure - even share some of their energetics, polar lows are an example of such a beast, and maybe it is unfair to exclude them based on their location of origin. I, however, don't sense a groundswell of opinion to strike the "originates over tropical or subtropical waters" from our definition. It has, on the whole, served us well.
So, take your pick of these ideas on what Thingamabobbercane was. We'll never know for sure, since there were no direct measurements of its structure by research aircraft. It will remain as a mysterious and beautiful example of the endless variety of weather on our planet.
There is nothing going on in the tropical Atlantic today, nor is there forecast to be any activity over the next six days. While this remains true, I will be posting blogs every Monday, Wednesday, and Friday.
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